In fluid media that are to be examined, in particular in gases or liquids, spectroscopic sensors are used to ascertain concentrations of particular substances through the absorption of IR radiation in relevant wavelength ranges.
Conventional spectroscopic sensors generally use a broadband infrared radiator and an infrared detector having one or more detector channels. Standardly, for each gas or liquid species a detector channel is used as measurement channel, and a further detector channel is used as reference channel.
In particular, two-channel detectors for detecting a particular substance are conventional. In such a two-channel detector, a first detector channel is used for the detection of the radiation in a relevant wavelength range, in which an absorption band of the substance to be ascertained or examined is situated, and the second detector channel is used as reference channel for detection in a broader wavelength range. The selection of the wavelength ranges takes place in general through the use of optical filters that are transparent for particular wavelengths, or wavelength ranges.
German Patent Application No. DE 10 2009 027 136 A1 describes a spectroscopic sensor that has an IR radiation source for emanating IR radiation, an absorption path for receiving a gas or liquid, at least one optical filter for the wavelength-selective transmission of the IR radiation moving through the absorption path, and at least one detector for receiving the IR radiation moving through the optical filter and outputting a measurement signal. The IR radiation source and the detector are fastened and contacted on a circuit bearer, and the absorption path is fashioned as the interior compartment of a reflector device that is fastened on the circuit bearer and that has a reflective interior surface, so that a direct transmission of radiation from the IR radiation source to the detector without reflection on the reflector device is prevented.
German Patent Application No. DE 10 2009 027 124 A1 describes a spectroscopic sensor that has an absorption path for receiving a substance that is to be examined, an IR radiation source for emanating IR radiation through the absorption path, a detector device that has at least one first and second detector element for detecting the IR radiation moving through the absorption path and for outputting measurement signals. An evaluation device is provided that receives the measurement signals of the at least two detector elements and ascertains a content of a first medium, the first detector element detecting IR radiation in a first wavelength range and outputting a first measurement signal, the second detector element detecting IR radiation in a second wavelength range differing from the first wavelength range and outputting a second measurement signal. In the first wavelength range there is situated an absorption band of the first medium and of a second medium. In the second wavelength range there is situated an absorption band either of the first medium or of the second medium, the evaluation device receiving the second measurement signal for correcting the first measurement signal.